Transpiration and the Ascent

232 7. Transpiration
per day. However, the actual evaporation usually is only about 80% of the theoretical
maximum because evaporation also is affected by the vapor pressure gradient
and by resistances in the vapor pathway. If evaporation exceeds the calculated
potential rate, it often is because incident energy is supplemented by
advection, the oasis effect shown in Fig. 7.2. Although the leaf area of plant
stands (leaf area index) usually is several times that of the soil on which they
grow, water loss cannot exceed that from an equal area of moist soil or a water
surface receiving the same amount of energy. As the soil dries, stomatal closure
reduces transpiration and drying of the soil surface reduces evaporation from
the soil, hence actual ET often is considerably lower than the potential rate calculated
from pan evaporation.
Evaporation and transpiration losses from a field of corn and a forest are
given in Tables 7.1 and 7.2, and the effect of removing plant cover in increasing
runoff and evaporation is shown in Table 7.2. One might expect water losses to
be similar from various types of plant cover receiving similar amounts of radiation,
but variations in reflectance (albedo), roughness of the canopy, depth of
rooting, and stomatal behavior result in significant differences (Rider, 1957).
An extreme example is shown in Fig. 7.13. Pineapple is a CAM plant (Joshi
- 3.0
-28
•.. -2 ..6
~ -2.4
E' -22
E -2.0
•...
-E - 1..8
!. -1.6
o -1.4
Ql
~ -1.2
,~ -1..0
~ -08
~ -06
59 V'l
o